Zachary Peter Gallen was born August 3, 1995 in Somerdale and grew up in Gibbsboro. His parents, Jim and Stacey, were both athletic, as was his older brother, Jay, who also starred in the local baseball leagues. A pitcher from the time he could hold a ball, Zac played a game with his dad called 3-and-0. He would start with a 3–0 count and try to throw three strikes in a row. If he threw a ball, the count would revert back to 3–0 for a new batter. At the age of 9 he led his Little League squad to the championship game, out-pitching the circuit’s top 12-year-old, 10–1.

Tall and powerfully built, Zac eventually mastered two off-speed pitches to go with a live fastball. He played in high school for Bishop Eustace Prep in Pennsauken, under coach Sal, Tropiano. As a senior in 2013, Zac was ranked in the Top 10 players in New Jersey.

The University of North Carolina offered Zac a scholarship and he grabbed it. He had been a fan of UNC baseball as long as he could remember. Zac’s freshman year for the Tar Heels saw him go 5–4 in 17 starts with a 4.64 ERA. His sophomore season in 2015 was a step forward as he lowered his ERA to 2.79. After pitching in the Cape Cod league that summer, Zac had another solid year as a junior, lowering his ERA even further. That season coach Mike Fox named him a co-captain.

The Cardinals selected Zac in the third round of the 2016 June draft. The team assigned him to its squad in the Gulf Coast League and he turned a 1.86 ERA in three starts and three relief appearances. Among his teammates was first-round pick Dakota Hudson. In 2017, Zac split the season between A-level Palm Beach, AA Springfield and AAA Memphis. He made 26 starts and went 10–8.

Zac was a key piece in a December trade with the Marlins, who traded slugger Marcell Ozuna to St. Louis. The other players were pitchers Daniel Castano and Sandy Alcantara, outfielder Magneuris Sierra. Zac spent the entirety of 2018 with New Orleans of the PCL, going 8–9 with 136 strikeouts in 133 1/3 innings. The Marlins were hoping he could pitch his way onto the roster in 2019, but he got shelled in Spring Training and was handed a ticket back to Triple-A. He was promoted him after he started the year with 9 wins in 10 decisions. Zac made his debut on June 20 against his old club and pitched five innings of one-run ball in a no-decision.

Zac got his first win as a major leaguer a month later against the White Sox. He matched Reynaldo Lopez through seven scoreless innings. The Marlins scored a pair of runs in the top of the eighth to win 2–0. Zac came to the majors with a low 90s fastball but a plus cutter, curve and change. He demonstrated control of all four pitches right from the start, racking up more than a strikeout an inning.

As the trade deadline neared, most experts believed Zac—sporting a sub-3.00 ERA—would be off the table. But the Marlins were in the market for a young shortstop and they found one in Arizona’s Jazz Chisholm. The price was Zac, even-up. He joined his new club on his birthday and was thrilled to be with a team in the playoff hunt. His catcher, Carson Kelly, was a familiar face from their days in the Cardinals farm system.